Geek asks and answers

What does this Haskell definition mean?

Eq a => [a] -> [(Int,a)]

I don’t know what Eq means or does it; I don’t know what the whole definition means?

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3 answers

This is the type restriction aused in the definition [a] -> [(Int,a)]: it says that it must define an instance for Eqtypeclass. In principle, instance types Eqprovide support for equality and inequality operators (==)and (/=).

More about product types here at learnyouahaskell.com . In nuce, typeclasses are somewhat similar to OOP interfaces.

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This means that the elements from [a] must be from the class Eq, which means that they must be checked for equality.

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Elements of type a must be of class Eq, as mentioned earlier. The Eq typeclass class defines types that can be compared for equality with other types, with th '==' operator / function

This means that your function takes as a single parameter a list of elements 'a'

[a] is a list of types a

and returns a list of (Int, a) pairs, i.e. a list of 2 tuples, where the first element in the tuple is Int, and the second is of the same type as the list passed to the function.

(not native English speaker here, sorry :))

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